The present invention relates generally to seismic shear wave generators, and more particularly, to a rotary actuated seismic shear wave generator and related testing methods.
Shear wave velocity is a valuable parameter for evaluating the behavior of subsoils, foundation systems, and earthen structures for both static and dynamic load conditions. The velocity characteristics of the upper 30 meters of soil, in particular, can have a great influence on the severity of earthquake ground motions, settlements, and stability of structures. Though there are several accepted methods for measuring shear wave velocity, direct-push downhole methods, such as the Seismic Cone Penetration Test, provide a fast, direct, and cost-effective means for determining shear wave velocity profiles of soil columns.
The downhole method requires a source for generating shear waves to be placed at the surface, and one or more receivers to be installed in the subsurface, beneath the source, at varying depths. Waves generated by the source at the surface, propagate into the subsurface where the motions can be detected and recorded with the aid of geophones, hydrophones, accelerometers, or some other transducers with means for detecting motion.
With the direct-push technique, the sensors are installed in a probe that is advanced into the soil without predrilling, typically at a constant rate actuated by hydraulic cylinders. The recorded signals at varying depths are analyzed to determine the travel times for the waves to propagate past one receiver and to the next. The difference in travel distances divided by the differences in travel times yields the velocity of the wave. By making measurements at several depths, a profile can be made of how velocities change with depth.
The most basic device for the purpose of generating horizontally polarized vertically propagating shear waves is a sledge hammer swung by an operator against a steel or wooden beam coupled to the ground surface by heavy weight. In order to eliminate the need for a hammer operator, devices have been developed that can be operated remotely by the person operating a receiver recording system. Existing remotely operated seismic source devices employ hydraulic, pneumatic, or electrical forces to accelerate a mass horizontally (parallel to the ground surface) into an object coupled to the ground, by heavy weight or other means. These devices are often very large, heavy, and expensive. Smaller, less expensive implementations of these devices depend significantly on the ground surface to provide the reaction force for accelerating the mass. The reaction forces to the acceleration of the mass are parallel to the ground surface and generate shear waves prior to the desired time at impact. The shear waves created by the acceleration reaction forces can obscure the shear waves created by the impact with the coupled object, making small inexpensive versions of the horizontal motion seismic sources inadequate.
For example, U.S. Pat. No. 5,486,026 discloses in its Abstract “An apparatus and a method for generating seismic waves utilizes a heavy flywheel positioned for rotation in a vertical plane to generate and store energy which is then transferred to the ground. The device includes two balanced hammers mounted on the flywheel as impact members. In operation, the flywheel is spun to a predetermined speed and then, on command, the hammers are extended outward from the flywheel so that one hammer strikes either a vertical or a horizontal strike member, thus providing a rapid transfer of a large amount of kinetic energy to the ground resulting in the generation of a set of seismic waves.” It is stated in the Summary of the Invention section of U.S. Pat. No. 5,486,026 that “there is provided an apparatus for generating seismic waves comprising a striker platform and a rotator assembly mounted on the striker platform. The rotator assembly comprises a flywheel positioned for rotation in a vertical plane, an impact means mounted thereon, and a means for rotating the flywheel. The impact means contacts a horizontal-impact assembly or a vertical-impact assembly to create the seismic wave.”
Conventional direct-push downhole procedures dictate that the progress of the penetrating sensor probe be halted during generation and recording of the shear waves. It would be desirable to have a rotary actuated seismic shear wave generator that improves upon conventional generators, such as those discussed above, in order to reduce time, effort, and cost associated with the testing procedure. If a source were able to repeatedly generate shear waves at a constant interval, the probe could continue to be advanced into the ground at a steady rate and record the detected motions without pause. Additionally, if the source were able to generate signals at a fast rate, overlapping and redundant measurements could be made to improve the reliability, accuracy, and resolution of the determined shear wave velocity profile.